Effluents from animal farms such as pig slurry, rich in the nutrient elements nitrogen, phosphorus and potassium “NPK”, are usually applied to agricultural land as fertilizer following spreading plans controlled by regulations. To limit pollution of soils, water basins and aquifers, authorized application rates are increasingly reduced.
This spreading can also achieve a concentration of said NPK elements on some seashores, which, with the heat, cause the proliferation of green algae, which are difficult to reuse because of their sea salt and sand loads.
Another use of these effluents is their treatment in dedicated dehydration facilities to obtain extracts concentrated in NPK elements, allowing delivery to external customers. However, these treatments require energy input and produce different materials that are not all used locally. The ecological balance may be poor, and the costs are not optimized.
A known process for treating animal waste, presented notably by document GB-A-2484530, achieves methanization of these effluents, producing mainly methane, carbon dioxide and heat. The materials derived from this methanization fuel a culture of microalgae to obtain upgraded products.
It is possible in particular to obtain, by means of a cogenerator using the methane, a production of electrical energy consumed on site or fed back into an electricity network, and of heat used for heating buildings or local activities.
Furthermore, another known treatment process, presented notably by document US-A1-2009/0294354, uses the liquid leaving the microalgae culture for feeding a culture of water plants in order to consume the nutritious products from this liquid.
A problem that arises with these methods is that some residual materials produced are not economically recoverable. In addition, these activities do not have a biological balance, and furthermore they do not consume all the main materials produced. It is then necessary to treat the residues that can cause processing, transport or disposal problems, increasing the overall cost of effluent treatment and degrading the ecological balance.
In particular, methanization and the cultivation of water plants produce sludge and a large amount of contaminated water, which must be removed, in particular by discharging the water or by spreading. However, these releases and this spreading may cause pollution of soil or water; they are thus subject to regulations and are increasingly limited.